The invention relates generally to improved underwater detection systems, and more particularly to a doppler sonar system employing intersecting, rotatable beams for detecting and tracking targets in flowing water.
Sound energy transmitted through a moving medium and reflected from a stationary object along the same path to a receiver located near the transmitter will experience no significant frequency shift since the doppler effects of the moving medium cancel out during transit in opposite directions. However, if the reflecting surface, for instance, an air-water boundary or floating particulate matter, is approaching relative to the transmitter-receiver array, the received sonic waves will contain doppler-shifted frequencies somewhat higher than the original transmitted frequency. Flowing water in a river normally exhibits certain discontinuities which form moving reflective surfaces and account for an ambient doppler spectra in the absence of target objects.
In the past, doppler sonar systems have been used in rivers to guard against sabotage of river installations by detecting the approach of swimming or floating objects. One type of equipment uses adjacent narrow bandpass filters to monitor the amplitudes of doppler shifts within certain frequency bands. The presence of an intruder swimming or floating downstream causes the amplitude to increase from the ambient level characteristic of the river current to an alarm level within one or more affected filters. The prior system employed a single pair of directional transducers which were mounted at the side of a river and oriented at a fixed angle to the primary current to provide a narrow-beam acoustic fence. While the beam position determined the bearing of a detected target, the prior device was not intended to provide ranging capability. Without range information, however, effective deployment of destructive countermeasures has been hindered especially at times of limited visibility. Moreover, since the beam was fixed, the indicated bearing would only be true at the moment of detection.
Using a single rotating or slewing array to update the bearing angle was not sufficient since range information would still be lacking. Moreover, downstream rotation of the single beam caused a nonlinear variation in the received frequencies since the doppler shift was a function of the cosine of the beam angle relative to the river current. Because of the resultant change in the ambient spectra, accurate detection with a rotating array of constant frequency would have required rapid, continuous adjustment of the alarm levels within the filtered bands.